This application is entitled to the benefit of the filing date of U.S. Provisional Patent Application No. 60/931,381 for “Estimating gas-oil ratio from other physical properties,” filed May 23, 2007. Knowing the gas-oil ratio (GOR) of crude oil formations is of considerable interest to those involved in the exploration and production of hydrocarbons (oil and gas). Various methods for estimating GOR of a fluid downhole based on performing infrared spectroscopy are already well known in the art. GOR is typically characterized in terms of a number of standard cubic feet of gas per stock tank barrel of oil. For black oils, GOR is typically less than 2000 standard cubic feet of gas per stock tank barrel of oil. For volatile oils, GOR is typically between 2000 and 3300 standard cubic feet of gas per stock tank barrel of oil. Gas and oil density and modulus, as well as oil viscosity, increase with molecular weight and pressure, and decrease with temperature. Gas viscosity has a similar behavior, except at higher temperatures and lower pressures, where the viscosity will increase slightly with increasing temperature. Large amounts of gas go into solution in lighter oils and substantially lower the modulus and viscosity.
Brine modulus, density, and viscosities increase with increasing salt content and pressure. Brine is peculiar because the modulus reaches a maximum at a temperature from 40 to 80° C. Far less gas can be absorbed by brines than by light oils. As a result, gas in solution in oils can drive their modulus so far below that of brines that seismic reflection “bright spots” may develop from the interface between the oil-saturated and brine-saturated rocks.
A prior reference of particular note with respect to pore fluids is Batzle and Wang, “Seismic Properties of Fluids,” Geophysics, v. 57, no. 11, pp. 1396-1408 (November, 1992) (hereinafter, “Batzle and Wang,” which is hereby incorporated by reference herein in its entirety for all purposes). The teachings of Batzle and Wang, commonly and collectively referred to as the Batzle and Wang relations, are widely known to and used by those of ordinary skill in the art.
In general terms, the Batzle and Wang relations comprise a series of separate correlation equations for sound speed and for GOR in terms of other parameters but it does not provide any equation for GOR in terms of sound speed, live oil density, pressure, and temperature. For example, one Batzle and Wang correlation equation relates gas-containing (“live”) oil density to GOR, gas density, and formation volume factor. Another Batzle and Wang correlation equation relates formation volume factor to GOR, gas density, stock-tank (“dead”) oil density, and temperature. The sound speed of live oil can be estimated by substituting for dead-oil density a pseudo-density based on expansion caused by gas intake into the equation for sound speed of dead oil. The sound speed of live oil at borehole temperatures and pressures is generally between 1100 and 1700 meters per second. Still another Batzle and Wang correlation equation relates the pseudo-density to formation volume factor, GOR, and stock-tank oil density.
Another reference, Han and Batzle, Velocity, Density and Modulus of Hydrocarbon Fluids—Data Measurement,” Society of Exploration Geophysicists Technical Program, Expanded Abstracts, 2000, pp. 1862-1866, doi:10.1190/1.1815792 (hereinafter, “Han and Batzle,” which is hereby incorporated by reference herein in its entirety for all purposes) elaborates on the Batzle and Wang formulations and is similarly widely known in the art.
While the utility of the Batzle and Wang approach to pore fluid characterization and the seismic significance of fluid and rock properties is widely recognized, there remain perceived shortcomings to such an approach, inasmuch as the suite of equations commonly ascribed to Batzle and Wang cannot be algebraically solved simultaneously to derive GOR values from sound speed, live oil density, pressure, and temperature because of their complexity. It is well-known to those of ordinary skill in the art that the roots of fifth-order or higher polynomials cannot in general be solved in terms of simple algebraic functions. This poses certain undesirable limitations on the practical utility of the prior art for this purpose of this invention as exemplified by Batzle and Wang and its progeny. For example, Batzle and Wang express GOR in terms of stock tank oil density and other parameters, which cannot be measured downhole.
In particular, it has heretofore not been shown a feasible methodology for characterizing the gas-oil ratios of a fluid downhole in terms of parameters, such as sound speed and live oil density, which are measurable downhole.